Sydney: Three people were dead and dozens trapped on the roofs of their homes in north-eastern Australia on Monday as severe floods swept through two states, inundating hundreds of properties and threatening thousands more.

The bodies of two men swept away by floodwaters were found on Monday, one in the Queensland state capital Brisbane and another further north at Gympie, following the earlier discovery of an elderly man's body at Burnett Heads near Bundaberg.

Queensland Premier Campbell Newman said four cities were facing major flooding in the wake of Tropical Cyclone Oswald with 1,200 homes already under water in Bundaberg where record floods had trapped at least 30 people on the roofs of their homes.

"We're going to try and rescue those people this morning," said Newman.

"Even though we have helicopters, the weather conditions are appalling and dangerous for flying. The floodwaters are very swift and it's also dangerous to go out in flood boats."

Police urged those trapped by the rapidly rising waters to retreat to their roof if safe and attempt to hail rescue helicopters flying over the town. About 100 troops had been deployed to help with rescues and two aircraft were en route.

Newman said the situation in Bundaberg, home to about 50,000 people 360 kilometres (220 miles) north of Brisbane, was "critical" on Monday. As many as 2,000 homes were expected to go under.

Deputy mayor David Batt said "hundreds of people, if not thousands of people, will be looking for evacuation in the next few hours" as the swollen Burnett River surged beyond nine metres (30 feet), breaking all prior records.

There was also flooding in Gympie, Maryborough and the Lockyer Valley, which was the epicentre of devastating floods two years ago that claimed 35 lives and brought Brisbane to a standstill for several days.

Brisbane was again expected to flood, with at least 3,000 homes and 1,250 businesses in danger. Almost 250,000 homes across Queensland were without power Monday.

Rural towns were also isolated by floodwaters in neighbouring New South Wales state, where hundreds of millimetres of rain was expected in coming hours including in Sydney.

Cyclones and floods are common in Australia's northeast during the warmer summer months.